Japanese engineers raced to prevent a meltdown at a stricken nuclear plant yesterday as rescuers scrambled to help millions left without food, water or heating by a devastating earthquake and tsunami.

A second explosion rocked the Fukushima nuclear complex and rapidly falling water levels exposed fuel rods in another reactor, but the United Nations’ nuclear watchdog said the crisis was unlikely to turn into another Chernobyl.

Rescue workers combed the tsunami-battered region north of Tokyo, where officials say at least 10,000 people were killed in the 8.9-magnitude earthquake and tsunami that followed it.

The big fear at the Fukushima complex, 150 miles north of Tokyo, is of a major radiation leak. The complex has seen explosions at two of its reactors, which sent a huge plume of smoke billowing above the plant.

The worst nuclear accident since the Chernobyl disaster in Ukraine in 1986 has drawn criticism that authorities were ill-prepared and revived debate in many countries about the safety of atomic power.

Switzerland put on hold some approvals for nuclear power plants and Germany said it was scrapping a plan to extend the life of its nuclear power stations. The White House said President Barack Obama remained committed to nuclear energy.

All reactors create energy through nuclear fission, which means that atoms split. When they split, they create heat and radioactive products. Even if you shut down the reactor immediately, as the Japanese did, it keeps producing heat. That’s why you have to cool the reactor.

So the real problem happened in the cooling of the reactor?

Yes. The tsunami that followed the earth­quake took out the generator that was supposed to cool the reactor.

Will Japan’s neighbors have to worry about nuclear radiation?

No. The wind has been blowing in the right direction, and the impact is small compared to what it would have been if the nuclear fuel had burned through the bottom.